Autoradiographic localization and characterization of [125I]Lysergic acid diethylamide binding to serotonin receptors in Aplysia

Using a radioactive form of LSD, researchers mapped serotonin receptors in the nervous system of the sea slug Aplysia californica. The radioligand bound to high-affinity sites (Kd = 0.41 nM) in the ganglia, with a distribution matching earlier studies. These sites differed from mammalian serotonin receptor subtypes. Guanine nucleotides reduced agonist but not antagonist binding, suggesting the sites are coupled to G-proteins. Autoradiography showed receptors mainly in the neuropil and some neuronal cell bodies. The method allows rapid characterization of receptor binding and distribution in single ganglia, enabling identification of individual neurons with specific receptor subtypes for correlation with physiological responses.

Study at a glance

Abstract

The sensitive serotonergic radioligand 2-[125I]lysergic acid diethylamide was used to study the distribution and pharmacological binding properties of serotonin receptors in Aplysia californica. The high specific activity of this radioligand allowed us to develop a methodology for the investigation of receptor binding properties and receptor distribution in a single ganglion. [125I]Lysergic acid diethylamide labels a population of high-affinity serotonergic sites (Kd = 0.41 nM) in Aplysia ganglia whose regional distribution matches that expected from previous electrophysiological and immunohistochemical studies. The properties of [125I]lysergic acid diethylamide binding sites in Aplysia are in general agreement with previous studies on [3H]lysergic acid diethylamide binding in this system but these sites differ from the serotonergic receptor subtypes described in the mammalian brain. Guanine nucleotides were shown to modulate agonist but not antagonist affinity for the [125I]lysergic acid diethylamide binding site in Aplysia, suggesting that this site is coupled to a G-protein. Images of serotonin receptor distribution in the Aplysia nervous system were obtained from autoradiograms of [125I]lysergic acid diethylamide binding. Serotonin receptors in ganglia tissue sections are located primarily within the neuropil. In addition, a subset of neuronal soma are specifically labeled by [125I]lysergic acid diethylamide. These studies indicate that [125I]lysergic acid diethylamide binds to sites in the Aplysia nervous system which display a regional distribution, pharmacological binding properties and evidence of coupling to a G-protein consistent with labeling of a subset of functional serotonin receptors. In addition, the techniques used in this investigation provide a general approach for rapidly characterizing the pharmacological properties and anatomical distribution of receptor binding sites in single invertebrate ganglia. Individual neurons containing these receptor subtypes can be identified by these methods and correlated with physiological responses in the same cell.